A conventional air/fuel control system for an internal combustion engine, as disclosed in JP-A-4-134159, is provided with an airflow meter in the intake pipe. A fuel injection amount is determined according to an amount of intake air measured by the airflow meter so that air/fuel ratio is controlled to predetermined value.
JP-A-7-119511 shows a vehicle driven by an internal combustion engine, the vehicle having a vacuum type brake booster which utilizes an intake pipe vacuum (negative pressure) created downstream of a throttle valve. The intake pipe vacuum is introduced into the vacuum type brake booster via a vacuum introducing passage connected to the intake pipe, and the pressure difference between the intake pipe vacuum and the atmosphere is utilized to amplify the depressing force of a brake pedal so as to increase the braking force of a brake.
In the vehicle having the intake pipe vacuum type brake booster mounted thereon, the intake pipe vacuum is introduced into the brake booster when the vacuum in the brake booster is reduced by a braking or the like while the internal combustion engine is running, so that the air in the brake booster is sucked into the intake pipe. However, the vacuum introducing passage of the brake booster is connected to the intake pipe downstream of the airflow meter (i.e., downstream of the throttle valve), so that the airflow to be sucked from the brake booster via the vacuum introducing passage into the intake pipe is not detected by the airflow meter. In the system for controlling the air/fuel ratio by setting the fuel injection rate on the basis of the intake airflow measures by the airflow meter, therefore, the airflow to be sucked from the brake booster into the intake pipe cannot be reflected on the air/fuel control when the intake pipe vacuum is introduced into the brake booster while the internal combustion engine is running. Therefore, the air/fuel ratio may be changed into the lean direction to lower the torque or increase the NOx emissions thereby to adversely affect the drivability or the exhaust emissions.
In JP-A-7-119511, the vacuum introducing passage of the brake booster is provided with a flow control valve for reducing the airflow to be sucked from the brake booster into the intake pipe, when it is decided on the basis of the running state (e.g., the running speed) of the vehicle that the depressing force of the brake pedal need not be amplified. Thus, the influences of the air to be sucked from the brake booster into the intake pipe are reduced to suppress the disturbances in the air/fuel ratio.
Generally, the brake booster is constructed in such a manner that the vacuum (or residual pressure) in the brake booster is kept by a check valve even after the internal combustion engine is stopped so that the pressure in the intake pipe restored the atmospheric level. If, however, the driver depresses the brake pedal while the internal combustion engine is stopped (or before the engine start), the residual pressure of the brake booster drops. Therefore, the residual pressure of the brake booster at the beginning of the engine start so seriously disperses that it may be left sufficient or substantially reduced close to the atmospheric level. When the internal combustion engine is started with the residual pressure of the brake booster being lowered, the intake pipe vacuum is introduced into the brake booster at the instant when the cranking is started to create the vacuum in the intake pipe. Then, the air in the brake booster is sucked into the intake pipe.
However, the airflow to be sucked from the brake booster into the intake pipe is not detected by the airflow meter, as described hereinbefore. In the system for controlling the air/fuel ratio by setting the fuel injection rate on the basis of the intake airflow detected by the airflow meter, therefore, the airflow to be sucked from the brake booster into the intake pipe with the engine at starting cannot be reflected on the air/fuel control. With the engine at starting, the air/fuel ratio may be changed into the lean direction by the influences of the air sucked from the brake booster into the intake pipe, to lower the torque or increase the NOx emissions thereby to adversely affect the drivability or the exhaust emissions.
As countermeasures, it is conceived that the aforementioned technique of JP-A-7-119511 is utilized to reduce the airflow to be sucked from the brake booster into the intake pipe at the starting time, by means of a flow control valve thereby to suppress the dispersion in the air/fuel ratio. However, the reduction of the airflow to be sucked from the brake booster into the intake pipe at the starting time by the flow control valve implies the retardation of the speed of introducing the intake pipe vacuum into the brake booster at the starting time. This further implies that the time period from the engine start to the restoration of the vacuum in the brake booster is elongated to make it impossible to restore the depressing force amplifying effect of the brake pedal early. Especially in case the residual pressure of the brake booster is substantially exhausted before the start, the aforementioned problems become serious if the airflow to be sucked from the brake booster into the intake pipe at the starting time is reduced by the flow control valve.